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Abstract

Swimming represents the primary means through which fishes interact with their
environment, and factors influencing swimming performance can profoundly affect their
distribution and survivorship. Traditional measures of swimming performance are
estimated under laboratory conditions using steady water flow. However, these
experiments potentially underestimate the actual cost of swimming under unsteady water
flows that characterize natural systems. Using a swimming respirometer and video
recordings, we swam Cymatogaster aggregata using a standard Ucrit swimming trial
under one of three flow conditions with the same mean water velocity at each speed
increment: steady flow (control), low amplitude water velocity fluctuations ( A=0.5BLs-1)
and high amplitude water velocity fluctuations (A=1BLs-1). We found that unsteady
flows increase the metabolic cost of swimming, but only when high flows push a
swimming fish beyond the threshold for exclusively aerobic metabolism (beyond Uburst).
Furthermore, unsteady flows at the highest amplitude treatment decreased the maximum
mean velocity (Ucrit) and mean gait transition velocity (Upc) achieved by individuals
compared to the control and low amplitude unsteady flows. However, lower costs of
swimming than predicted in low amplitude flows below Uburst suggest that fish are able to
take advantage of the cyclical wave patterns and economize energy expenditure. Mean
pectoral fin beat frequency did not differ significantly among treatments. This is the first
study exploring the costs of swimming under unsteady flow in a marine labriform
swimmer.